This invention relates to apparatus and methods for controlling the response of a wave energy converter (WEC) so as to increase the efficiency of the WEC in producing energy. The invention also relates to control systems for wave energy conversion devices, and more particularly to a control algorithm for capturing wave energy more efficiently.
Wave energy converters (WECs) of interest generally include first and second bodies which can move relative to each other when placed in a body of water and subjected to the forces of the waves. Typically, one body, referred to as a float or shell, is designed to move in phase with the waves and the other body, referred to as a column or spar, is designed to either move out of phase with the waves (resulting in a two body system) or to be held relatively fixed (resulting in what may be termed as a one body system). A power conversion mechanism (or power take-off, PTO, device) is connected between the two bodies to convert their relative motion into a form of useful energy (e.g., electrical). Typically, WECs of interest include a controller and data processor which may be a part of, or separate from, the PTO for adjusting the operation of the power take-off (PTO) device.
A PTO for use in practicing the invention may include any suitable device or component (e.g., a motor/generator, a linear electric generator, a rack and pinion, or any other mechanisms or linkages) connected between the first and second bodies of a WEC capable of converting their relative motion into useful energy (e.g., electric power). The performance and functioning of the power take-off (PTO) is critical to harnessing wave power in an efficient manner. Generally, the dynamic behavior of a WEC's PTO is governed by a combination of environmental factors and control factors. The environmental factors include wave- and wind-induced forces acting on the bodies of the WEC. The control factors are governed by the WEC's controller and data processor which may be used to adjust force(s) applied to, and between, the first and second bodies of the WEC.
A WEC system can be modeled to generate a mathematical formulation which can be used to predict the responses of the WEC and the PTO. Different WEC and PTO systems may respond in different ways. The way a particular WEC and its PTO respond to various stimuli can be predicted. Thus, the response of a PTO to the relative movement of the two bodies (e.g., float and spar) can be defined or modeled. Likewise, the response (movement) of the WEC to energy supplied to the PTO can also be defined or modeled. The better the model, the more accurate is the mathematically predicted response of the WEC and PTO. The mathematical prediction(s) may be used to control the tuning of the PTO and/or the positioning of the PTO and/or the infusion and extraction of power to and from the PTO to increase the efficiency of the system. This presumes the incorporation of a controller and data processor coupled to, and between, the components (e.g., float, spar, PTO) of the WEC. The behavior of the PTO connected between two bodies which move relative to each other in response to motion of the waves and which converts their relative motion into useful energy is significant in harnessing wave power in an efficient manner.
Typically, power is extracted from the PTO. But, at times, it is desirable to supply power to the PTO and cause the PTO to control or adjust the relative motion of the two bodies to optimize the average power obtained from the relative motion of the two bodies (float and spar). As noted above, the two bodies may be designed to move, or only one may be designed to move with the other body being held fixed.
To optimize the power generated by a WEC, the WEC system may include a wave sensor located “upstream”, at some distance from the WEC, to sense selected characteristics of an incoming wave and provide the WEC with “predictive” information as to the incoming wave. The predictive information is desirable since, due to properties of moving floating bodies, optimal power capture requires motion of the WEC and PTO with regard to waves arriving some seconds later. Thus, systems of interest include a wave-sensing (generally external to the WEC) and prediction subsystem for sensing selected characteristics such as the amplitude (elevation) and frequency of incoming waves in order to determine the nature of the waves predicted to impinge on the WEC, and a prediction subsystem. The wave prediction subsystem supplies its signals to a control computer which then controls or adjusts the PTO (which may include supplying and/or extracting power from the WEC) in an effort to try to optimize the average power output of the WEC.
However, a problem exists in that the predicted wave ΨP may differ from the actual wave ΨA which impinges on the WEC. The difference may be due to several different factors. One factor is that the wave-detection sub-subsystem of the wave prediction subsystem is prone to inaccuracies in sampling, which leads to prediction errors. Also, there are many different currents and atmospheric conditions which cause the actual wave incident on the WEC to be different form the predicted wave. The difference between a predicted wave ΨP and the actual wave ΨA may be defined as “wave error.” If, and when, an actual wave ΨA, different from the predicted wave ΨP, impinges on the WEC, the WEC components (e.g., the PTO and floating bodies) may not be positioned to their optimum position for maximizing response to the impinging wave (and in fact may be positioned significantly away therefrom). The result is that the response of the WEC is less efficient.
In a known prior art method full dependence is placed on the information corresponding to the predicted wave and the assumption that the predicted wave will impinge on the WEC. This gives rise to significant errors when the wave which actually impinges on the WEC is different than the predicted wave. Therefore, the prior art scheme is not satisfactory. Furthermore, there is no known economically viable system which can provide sufficiently accurate wave prediction. Thus, a problem with known predictive systems occurs when there is a wave error (e.g. when the wave impinging on the WEC is different than the predicted wave.)